The clutch switch is not called out as a separate part, but part of the lever assembly;
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your best bet is to look for a used one - most of the Triumph range uses the same switch and splice it into the harness.
Is the housing itself broken? (which the switch mechanically plugs into?); if the perch part is broken, you may as well look for complete left switch assembly.
Here's a good-looking R3 switch - only the switch however, the clutch perch (which includes the mechanical socket into which the clutch switch locates) is not included
LEFT Handlebar Switch Triumph Rocket III 05-12OEM